The field of extrusion, and more particularly, the field of thermoplastic foam extrusion, entails many techniques that are utilized in order to generate a uniform thickness product having desirable surface finish features and appropriate dimensional qualities. The ability to fine tune the dimensional qualities and surface finish on a sheet that is generated with an extruder die is a desirable feature.
FIG. 1 illustrates one construction for an extruder die assembly that was implemented in a single commercial device as well as a single test device in the mid-1980s by the present Applicant in an attempt to improve the surface finish and dimensional qualities when forming polymer foam sheet from an extruder die.
As shown in FIG. 1, an annular extruder die assembly 10 is shown mounted to the downstream end of an extruder 12 (similar to extruder 100 shown below with reference to FIG. 2). Die assembly 10 includes a barrel/spider/die assembly 16 comprising a barrel 18, a spider 20, and a die 22 that are joined together and mounted onto an end 14 of extruder 12. A barrel heater spacer 24 is provided about barrel 18. Similarly, a spider clamp ring 26 mounts spider 20 onto barrel 18. A spider heater spacer 28 is mounted about spider 20. A die lip clamp ring 30 mounts die 22 onto spider 20. Additionally, a sheet metal shell 32 forms an enclosure about spacer 24, ring 26, spacer 28, and ring 30. In this manner, a cooling air passage is provided within shell 32 from a source of cooling air 36 that passes through spacer 24, ring 26, spacer 28, and ring 30 where the flow of air exits adjacent an air ring 34. Air ring 34 provides an annular flow of cooling air onto an outer surface of a tubular film of extruded foam material that is being ejected from an annular extruder die.
Spider 20 includes a spider core 38 configured to support a die shaft 42 downstream and coaxially with respect to a bore 40. Spider core 38 is supported by a single radial arm 39 within bore 40. Die shaft 42 mounts onto core 38 via a male threaded portion 44 that is received into a complementary female threaded portion 46 of core 38. An outer die lip collar 48 is seated onto a cylindrical mounting post 50 of shaft 42 via a washer 52 and a jam nut 54. Jam nut 54 is rotated (or loosened) in order to separate collar 48 from an inner die lip 62 when it is necessary to enlarge a die gap between the inner die lip 62 and an outer die lip 64 of collar 48 to clean particles from the die gap.
According to the prior art construction of die assembly 10 of FIG. 1, a plurality of die lip adjuster brackets 56 are mounted in equally circumferentially spaced-apart positions about die 22 via individual threaded adjuster bolts 58. Bolts 58 are each received into a threaded bore 60 in die 22. Accordingly, it is necessary to machine bores 60 into die 22 (which is typically made from hardened steel) at various circumferentially spaced-apart locations thereabout. Bolt 58 is tightened into threaded engagement within bore 60 in order to drive Z-shaped bracket 56 down a frustoconically shaped slope face of die 22 which drives a finger of bracket 56 downwardly and forward into engagement along a radial surface of inner die lip 62. Accordingly, a die gap dimension between lips 62 and 64 can be adjusted at various circumferential locations thereabout by adjusting each respective bolt 58 for each bracket 56 at each circumferential location about die 22.
Several problems are presented by the construction depicted in FIG. 1. First; threaded bores 60 have to be machined into die 22 at various locations. Secondly, another problem is created in that brackets 56 do not apply a bending force onto inner die lip 62 at a constant radial location. The force applied from bracket 56 onto lip 62 acts on a variable fulcrum length when adjusting the die gap between lips 62 and 64. Hence, the radial repositioning of bracket 56 relative to lip 62 imparts a second variable (over the displacement of the fastener) that complicates the adjustment process and makes it more difficult to achieve a desirable and carefully controlled die gap between die lips 62 and 64 at each of the circumferential locations about die lips 62 and 64. It has been found through experience that improvements are still needed in order to increase the controllability and precision available when adjusting the tailoring of a die gap about an entire circumference of an extruder die in order to achieve improved dimensional qualities and surface finish (inner and outer) on a polymer foam sheet of material that is extruded therefrom.
Feedback from the above-described prior art effort identified the need for further improvements in order to further enhance the surface finish and dimensional qualities of a polymer foam sheet being formed by an extruder die. Accordingly, the Applicant has implemented further improvements which are described below in the Detailed Description.